I have heard from many places that if you have more than one BRD/ or /BRD in pt then you can have more than 2 songs active on the party - although it will be the last two songs cast from every BRD/ and the last song cast from any /BRD - as long as the songs stack with each other...

In this PT: -X5 gets to cast two songs on the PT (and can use instruments to increase strength or duration) -X6 (odd job combo but I have seen it occasionally) gets to cast one song on the PT, but it cannot be the same as one of the songs that X5 did or it will overwrite. Also, X6 cannot use an instrument and can only cast songs available to the subjob level.